This book defends the derivation of the ethical principle of universalizability presented by Jürgen Habermas, and illustrates the importance of this principle for both social science and social policy. Beginning with a comprehensive analysis of social theory, Smith divides the theory into nine distinct branches, each devoted to a fundamental question; three branches fall under social science, three under social ethics, and the last three make up social policy. He shows in detail how each branch has its own methodologies and basic categories, while being systematically connected to the others as well. Next, he makes a compelling defense of Habermas's main contribution to social ethics and contrasts Habermas's rational foundation for ethics with the decisionism defended by Max Weber. Smith then examines the implications of the principle of universalizability for a number of important issues in social theory.